1. Field of the Invention
This invention relates to a blade for use in eye surgery and a method for performing a surgical procedure on a cornea of the eye. More particularly, this invention relates to a marking device for marking the cuts to be made in the eye, a blade with a planar, dull distal edge and a method for deepening incisions in the cornea of the eye using the blade of the present invention.
2. Description of Related Art
Refractive errors, such as myopia (nearsightedness) and astigmatism, are commonly corrected with eye glasses or contact lenses. However, radial and astigmatic keratotomy offer an alternative method of vision correction for many people with refractive errors who are not satisfied with glasses or contacts. Performed since the early 1970s, radial keratotomy (RK) has generally produced good results in the permanent correction of myopia. A modification of the RK procedure, astigmatic keratotomy (AK), generally provides permanent correction of astigmatism.
A refractive error is a defect in the way light passes through the eye. In normal vision, light enters the eye through the cornea and is focused at a single point on the retina at the back of the eye. With a refractive error, light rays do not refract (bend) properly to achieve a single focus point. Instead, light rays focus in front of the retina, behind the retina, or at two different points. Refractive errors usually result from a defect in the length of the eye or shape of the cornea.
Myopia is a condition in which close objects are seen clearly but distant objects appear blurred. Nearsightedness results from a lengthened eyeball (which is more oval than round) or a cornea that is curved too steeply. The increased length of the eye or the steep curve of the cornea causes the light rays to be focused in front of the retina.
Astigmatism, a condition in which objects at all distances appear somewhat indistinct and distorted rather than sharp and clear, is caused by a misshaped cornea. Because the curve of the cornea is uneven or warped instead of an even curve, the light rays are not refracted equally in all directions. As a result, a single focus point on the retina is not achieved and distorted vision occurs.
Keratotomy is a generally effective surgical technique used to correct or reduce nearsightedness and astigmatism. The procedure involves the strategic placement of tiny incisions on the surface of the cornea. Radial and astigmatic keratotomy alter the shape of the cornea so that light rays focus precisely on the retina.
To correct nearsightedness, the surgeon makes radial incisions in the outer surface of the cornea, while looking through a high power surgical microscope. The incisions do not go through the central cornea. A clear ring is left centrally as the "optical zone." The incisions are made in a pattern similar to the spokes on a wheel, with the central optical zone on the hub.
AK is very similar to RK. However, the pattern of the incisions is modified to achieve the astigmatic correction. Horizontal or curvilinear incisions are used to correct astigmatism. A combination of both radial and astigmatic keratotomy can be used in a single procedure in patients with both myopia and astigmatism.
Before surgery, thorough testing is performed to determine the amount of myopia and/or astigmatism and the diameter, thickness, curvature and health of the cornea. From this information, a surgical plan is developed. The exact number, depth, length and location of the incisions must be carefully planned to achieve the desired flattening and optical correction.
The precise incisions on the outer portion of the cornea allow the center of the cornea to flatten. When the incisions heal, the cornea has a different shape, a flatter center. The flattening of the cornea moves the focal point closer to the correct position on the retina, similar to the way glasses and contact lenses correct the path of light rays as they enter the eye.
Exact incisions having a precise depth are important so that the correction of curvature of the cornea is not too great or not great enough. In RK or AK surgery, the amount of eyesight correction achieved is directly related not only to the length of the incisions but also to the depth of the incisions made by the surgeon. However, as with any surgical procedure, complications, such as perforation of the cornea, occur. It is also possible that no visual improvement may result or vision may improve only temporarily so that a second procedure may be required.
To perform keratotomy, a knife blade is held by a handle which normally includes a pair of footplates positioned on either side of the blade, and are designed to slide along the outer surface of the cornea. The location of the footplates relative to the tip or point of the blade can be adjusted by a micrometer setting to control the depth of the cut.
Since keratotomy involves cutting the cornea of a human eye, great care must be taken to make sure that the cuts are precise and at the depth necessary to provide for the indicated correction. Consistent results are difficult to achieve.
At this time, there are instruments that exist that fairly accurately measure the length and number of corneal incisions. There are also instruments that measure the thickness of the cornea. These instruments are called pachymeters. However, there is no device or method currently available for ensuring the precise depth of an incision as it is made by the surgeon. This problem is made even more difficult because the thickness of the cornea changes radially from the central portion of the cornea to the peripheral portion of the cornea. The ability to ensure the exact depth of the incision is critical because an operation on the cornea is only successful if the incisions are cut into 90% or more of the depth or thickness of the cornea, but less than 100% of the thickness of the cornea for the entire length of the incision. Obviously, if the incision is too deep, the cornea may be perforated which can lead to severe complications. In the alternative, if the incisions are too shallow, the net effect of the surgery fails to correct the problem.
Presently, micrometer knives with diamond cutting blades are used to cut incisions in the cornea. These blades are very sharp along the edges and merge at their tip to form a sharp point. Since the exact depth of the cut cannot be positively known or observed during surgery, even if the surgeon is highly skilled, the depth of the cut may be too shallow or too deep which in either case results in an unsuccessful operation. Accordingly, cuts are normally made conservatively because the knife if extremely sharp and many surgeons tend to be over-cautious, especially, when the knife is close to the optical zone. As a result, when the incisions in the cornea are not deep and precise enough, then the problem which has been known to occur is an undercorrection of the myopia, for example.
Moreover, when the tip of this diamond blade approaches cutting 90% of the thickness of the cornea, the risk of perforating the cornea is high due to the sharpness of the blade tip. With such a sharp point it is easy to prick completely through the cornea. The risk of perforating the cornea further increases when attempts are made to cut the incision beyond 90% of the thickness of the cornea. As a result, it is difficult, if not impossible, to obtain an incision that extends into 90% or more of the thickness of the cornea, while, at the same time, avoiding perforation of the cornea.
In addition, with this sharp pointed tip blade it is difficult for a surgeon to control the consistent depth of the blade even when footplates are used. In particular, since the tip of the blade terminates in a point, it is easy for the tip to penetrate further into the depth of the cornea with an unintentional slight increase in pressure by the surgeon. Moreover, it is difficult for a surgeon to obtain a smooth even cut bottom. Because the surgeon is moving a pointed tip along the cornea, it is likely that the tip of the blade in combination with an unsteady surgeon's hand will cut an incision with an uneven bottom. When the bottom of the incision becomes unsmooth or wavy the success of the surgery is compromised.
Another problem arises when the knife blade was not properly set during cutting of the incision and the incision is cut too shallow, for example. In such case, it is difficult to retrace the cut at the proper blade setting without straying from the exact path of the initial cut with the sharp diamond blade. Additionally, it is difficult to enhance or deepen an existing incision without the fear of perforating the cornea.
Moreover, the problem of glare exists in this type of surgery. In particular, the incision made in the cornea of the eye can reflect light causing glare. When incisions are cut too close to the optical zone of the eye, the glare caused by the incision frequently reflects into the optical zone of the eye. Glare can compromise the patient's vision as well as be a nuisance to the patient.
Accordingly, the need exists for an apparatus and a method for cutting and ensuring the precise depth of an incision being made into the thickness of the cornea, as well as, a constant cut into 90% or more of the cornea along almost the entire length of the incision. More particularly, what is needed is an apparatus and method for reliably and safely cutting or enhancing incisions in the cornea at a depth of 90% or more of the depth of the cornea without the risk of perforating the cornea.